Rectifiers of different designs may be used for supplying DC systems from AC systems. In motor vehicle electrical systems, bridge rectifiers in a six-pulse version are frequently used in accordance with the three-phase AC generators typically installed therein. The present invention is similarly suitable, however, for bridge rectifiers for other numbers of phases, for example, for four-phase or five-phase generators.
One critical operating condition in the case of bridge rectifiers is load shedding (load dump). This occurs if, in the case of a highly excited generator and a correspondingly high emitted current, the load on the generator or the bridge rectifier connected thereto (for example, due to shutdown of consumers) is reduced suddenly and the load cannot be sustained by capacitively acting elements in the DC voltage network (for example, the battery in the motor vehicle electrical system). In this case, as a result of the energy which was not dissipated in the generator, a high voltage could still be supplied into the motor vehicle electrical system via the bridge rectifier connected to the generator, in the extreme case up to a period of approximately 300 ms to 500 ms. This energy therefore generally has to be able to be sustained in the bridge rectifier to protect electrical components in the motor vehicle electrical system from damage caused by voltage surges. This is generally carried out in passive bridge rectifiers by the rectifier Zener diodes installed therein, in which the voltage surge may be clamped and the excess energy may be converted into heat.
As explained in DE 10 2009 046 955 A1, for example, the use of active bridge rectifiers is desirable in motor vehicles, because, among other things, they have lower power losses in comparison to passive or uncontrolled bridge rectifiers. Presently available activatable or active switching elements for such active bridge rectifiers, for example, MOSFETs, do not have an integrated clamp function with sufficient robustness and are not able to sustain the voltage surge. Therefore, additional protection strategies are required in active bridge rectifiers.
In the event of load shedding, for example, the generator phases may be short-circuited by briefly switching all switching elements of the upper or lower rectifier branch into the conductive state, as discussed, for example, in DE 198 35 316 A1 and discussed in DE 10 2009 046 955 A1. This takes place in particular on the basis of an analysis of the output voltage applied to the DC voltage terminals of the active bridge rectifier. If it exceeds a predefined upper threshold value, a corresponding short-circuit is initiated and the output voltage drops. If the output voltage thus falls below a predefined lower threshold value, the short-circuit is canceled and the output voltage rises again. It is therefore typical hysteresis behavior. The output voltage therefore essentially swings between the upper and the lower threshold values in the event of load shedding.
Problems may arise here in so-called decentralized active bridge rectifiers, in which the individual half-bridges each have independent control circuits, which each detect the output voltage individually. Such half-bridges including independent control circuits are also referred to within the scope of this application as rectifier modules or phase modules. Since certain tolerances are unavoidable in these control circuits, different switching behavior may take place in the individual half-bridges, as explained below. This relates in particular to control circuits which are implemented with the aid of application-specific integrated circuits (ASICs). Individual switching elements in the active bridge rectifier may be significantly overloaded by the differing switching behavior, which may result in thermal destruction of the corresponding switching elements and a failure.
The demand therefore exists for improved protection strategies for active bridge rectifiers in the event of load shedding.